The invention relates to a spectrometer comprising means for directing radiation emanating from or passing through a sample onto a radiation detector, signal processing means for processing the signal produced by the radiation detector, and display means for displaying the processed signal, wherein the detector comprises an m column by n row array of charge coupled devices, means for accumulating the charges from a plurality of devices on an output capacitor, and means for producing an output signal representing the charge on the output capacitor.
Such a spectrometer is described in the following three papers.
1) Charge Transfer Device Detectors for Analytical Optical Spectroscopy--Operation and Characteristics by R. B. Bilhorn, J. V. Sweedler, P. M. E. Epperson, and M. B. Denton; published in Applied Spectroscopy, Volume 41, Number 7, 1987, pages 1114 to 1125:
2) Intensified charged-coupled-device cameras for a spatially resolving extreme ultraviolet spectrometer by D. Content, M. Perry, D. Wroblewski, and H. W. Moor; published in Optical Engineering, Volume 26, Number 8, August 1987, pages 806 to 812: and
3) Optimising charge-coupled-device detector operation for optical astromony by Robert W. Leach; published in Optical Engineering, Volume 26, Number 10, October 1987, pages 1061 to 1066. They all refer to the advantage of charge coupled device (CCD) detectors in that a read-out mode in which the charge from more than one detector element can be combined within the CCD before being read out. This process is called binning. The advantage of summing the analogue signal "on chip", as compared with digital summing in memory, is that the summed charge is subject to only one read operation and as a result has only the noise associated with one read operation; whereas digitally summing the data also sums the noise associated with each read out operation.
This technique can only be applied where either a small number of detector elements which accumulate a maximum charge are binned or if a large number of detector elements are binned each detector element can only accumulate a small charge. This is because in one direction of the array the shift register can only hold a small amount of charge, typically between one and five times the maximum detector element charge, and in the other direction the output capacitor has a small charge handling capacity, again typically not more than five times a shift register stage charge.